Path: utzoo!attcan!uunet!husc6!cfa!ward
From: ward@cfa.harvard.EDU (Steve Ward)
Newsgroups: comp.sys.ibm.pc
Subject: Re: Usable I/O Address range on the PC bus ...
Keywords: Limited to addresses up to 0x0400 only??
Message-ID: <1279@cfa.cfa.harvard.EDU>
Date: 22 Nov 88 15:26:28 GMT
References: <5065@whuts.ATT.COM> <4229@encore.UUCP> <1275@cfa.cfa.harvard.EDU> <23609@amdcad.AMD.COM>
Organization: Harvard-Smithsonian Ctr. for Astrophysics
Lines: 18


Aha!  I finally understand what Phil has been saying.  Correct me if I
am wrong, but what you are saying is that using 16-bit I/O address
decoding, garner a set of (up to 64) I/O bytes by using I/O bytes
spaced 1024 (1K) byte addresses apart.  By mapping the lowest address
into the bottom 1K address space, you guarantee that the upper
(normally 10-bit decoding "ghosted" addresses) addresses on 1K
increments are available for 16-bit decoding.  Why didn't you just
say that? :-) :-)  (now that I have the cotton out my ears, I can
hear just fine!  :-) )

This will work just fine.  I guess I just had my mind set on a
block of contiguous addresses.  However, the real problem remains:
lots of 10-bit decoded boards really crowding the bottom 1K I/O
space (and hence crowding the whole 64K).  However, your suggested
method stretches the I/O ports as far as they can go, under the
circumstances.
ward@harvard.edu